1. Field of the Invention
The present invention relates to a power source apparatus that connects the vehicle driving battery to the car-side load via contactors (high power, electric automotive-grade relays), and in particular relates to a car power source apparatus that prevents the detrimental effects of fused contacts.
2. Description of the Related Art
The car power source apparatus has first and second contactors that connect to the positive and negative output-side of the battery. In this power source apparatus, contactors are switched ON to connect the battery with the car-side load. When the vehicle is not in use, for example, when the automobile ignition switch is OFF, the contactors are switched OFF. Furthermore, if the automobile has an accident or collision, the contactors are also switched OFF to cut-off battery output and increase safety.
A capacitor with a large capacitance value is connected in parallel with the car-side load that connects with the power source apparatus. Its purpose is to instantaneously output high power. The capacitor is charged by the battery. Since the capacitance of the capacitor is high, charging current becomes extremely large when the capacitor is completely discharged. Consequently, when the contactors, which connect to the positive and negative output-side of the battery, are switched ON, extremely large transient charging current can flow. Large charging current can damage contacts on the contactors. In particular, contactor contacts can fuse together due to large charging current. If the contacts fuse together, the contactors cannot be switched OFF and the driving battery cannot be disconnected from the load. To prevent this negative result, a power source apparatus has been developed that is provided with a pre-charge circuit to pre-charge the capacitor before turning the contactors ON (refer to Japanese Patent Application Disclosure 2005-295697).
As shown in FIG. 1, the car power source apparatus described in JP 2005-295697 is provided with a pre-charge circuit to supply auxiliary charge to the capacitor. The pre-charge circuit is connected in parallel with the first contactor on the positive-side of the battery and supplies auxiliary charge to the capacitor while limiting current. This pre-charge circuit is provided with a current-limiting pre-charge resistor, and a pre-charge relay connected in series with the pre-charge resistor. In this power source apparatus, the second contactor connected to the negative-side of the battery and the pre-charge relay are switched ON to pre-charge the capacitor. After the capacitor is pre-charged, the first contactor is switched ON to connect the battery with the car-side.
In the power source apparatus shown in FIG. 1, if the car-side becomes short circuited, extremely high current, for example 1000 A of short circuit current, can flow through the contactors. High short circuit current is a cause of contactor contacts fusing together. In particular, if the contacts “chatter” under short circuit conditions, they can easily become fused together. Contactors are switched ON and OFF each time the car ignition switch is turned ON and OFF. Further, since the contactors switch high currents, a long-lifetime durable metal such as tungsten is used as the contact material. For example, currently adopted contactors utilize durable contacts having fixed contacts of copper, and moveable contacts of tungsten laminated on a copper surface. Durable contacts demonstrate superior long-lifetime characteristics when switched ON and OFF with high currents. However, contactors provided with durable contacts cannot effectively prevent fused contacts due to high current flow from a car-side short circuit. This occurs because, if contact “chatter” occurs under high current conditions, tungsten, which is a durable contact material, will be heated to high temperature and will melt copper.
Fused contacts due to short circuit current can be prevented with high-capacity type contacts having fixed contacts and moveable contacts made of copper. However, while high-capacity type contacts made of copper can prevent short circuit induced fused contacts, they have the drawback that long-lifetime cannot be achieved. Copper, which is a high-capacity type contact material, shows low contact temperature rise due to short circuit current flow, but copper has a low melting point. Tungsten, which is a durable contact material, shows high contact temperature rise due to short circuit current flow, but tungsten has a high melting point. Consequently, in a contactor having fixed contacts of copper and moveable contacts with tungsten on the surface, the tungsten will be heated to high temperature by short circuit current and copper will melt causing the contacts to fuse together. Contactors with fixed and moveable contacts of copper show less contact temperature rise due to short circuit current and can better prevent fused contacts than contactors with tungsten contacts, but they cannot achieve long-lifetime. Therefore, to implement long contact lifetime, the prior art car power source apparatuses have the drawback that fused contacts due to car-side short circuit cannot be prevented. If contactor contacts become fused together, battery output cannot be cut-off and it is difficult to insure safety. A car power source apparatus that can reliably switch OFF contactors to cut-off battery output under abnormal conditions is in demand. While prior art power source apparatus contactors can achieve long-lifetime, prevention of short circuit current induced fused contacts is extremely difficult. Prior art power source apparatus contactors have the drawback that long-lifetime and prevention of fused contacts are mutually exclusive characteristics that cannot both be satisfactorily realized.
The present invention was developed to resolve these drawbacks. Thus, it is a primary object of the present invention to provide a car power source apparatus that can reliably cut-off battery output in abnormal circumstances while achieving long contactor lifetime.